A wideband differentially-fed slot antenna is presented for millimeter-wave (mmWave) applications. A novel method of using stepped corner-shaped slot is first utilized to establish the wideband circularly polarized (CP) radiation. In the configuration of corner-shaped slot, two wide open slots at the ends are utilized for effective orthogonal radiation, while the narrow slot at the center is utilized for power transmission and quadrature phase delay. An equivalent circuit is given to illustrate the inner working principle for CP radiation. In addition, square cuts are etched on the four corners of the radiating patches to further increase the axial ratio (AR) and impedance bandwidth. Based on this design concept, the antenna element was first designed and fabricated for performance verification. Then, a 1×4 linear array with beam scanning performance and a 4×4 planar array with high gain and stable radiation were designed and fabricated. Both the simulated and measured results show that the 1×4 linear array and 4×4 planar array can have wide overlapped impedance and AR bandwidths of 30.6% and 33.6% with thickness of 0.16λ0. The advantages of compact size and wide bandwidth make the presented antenna a good candidate for mmWave applications.